Power amplifier



Oct. 19, 1937. w VAN B. ROBERTS 2,096,561

POWER AMPLIFIER Y Filed Aug. 7, 1934 I 73 1 1650 CLASS .B l 4 12 5 6 RECEIVER r WITH IIUTOH/ITIC MEN/VS f- 21 153 20 U 2 c |---fl 31 2a 27 Z2 2 4 6 F A ZERO Ems 40 a8 cLnss B '34 4a l 39 36 42 37 f i z 3 45 44 1 I v I 42 43 SIG/VAL l E; j l souece 0F suBsrM r/Aar 1 u a l co/vsrmvr I AMPLITIIDE 52 6'5 .4. A B 47 i- 0 I IN VEN TOR ViznliHoberts 7' I "ATTORNEY Patented Oct. 19, 1937 UNITEDQSTATES POWER AMPLIFIER Walter van B. Roberts, Princeton, N..J., assignor, by mesne assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Appiication August 7,

17 Claims.

The present invention relates to electric signal amplifiers of the type employed in radio receiving apparatus and the like for audio frequency amplification. Such amplifiers may include one or more voltage amplifier stages and a final or output amplifier stage, generally termed a power amplifier, having relatively higher power handling capacity.

Amplifiers of the above character may be classified, in'accordance with the mode of operation and power handling ability, as class A or as class B amplifiers, and each has certain desirable operating characteristics.

The more comrnon class A amplifier is characterized by the fact that the average value of plate or anode current for the electric discharge devices or tubes employed therein, does not Vary appreciably when a signal is impressed on the tube grids, that a fixed negative grid bias is employed, and that the output alternating current voltage bears a linear relation to the input alternating current voltage. The grids are not driven into the positive range of operation except on overload. This type of amplifier has the advantage of high. fidelity at low volume or signal amplitudes for audio frequency amplification.-

The class B amplifier is characterized by the fact that the power output is proportional to the square of the excitation grid voltage. The eleciii trio discharge devices or tubes employed therein are biased to essentially plate current out 01f or slightly above, and the average value of direct late current varies with the input alternating current voltage. The plate current flows only during the positive swings of the input voltage. The grids may driven positive until the output voltage begins to deviate from a linear relation with respect to the inputvoltage. U The class B amplifier is therefore adapted to handle relatively large amounts of power economically, since the plate current is normally substantially zero or relatively low, and when arranged as an improved or audio frequency class B amplifier, having the amplifier tubes in balanced or pusl1p11sh relation, and with low impedance input and output circuits, may advanously be employed as a power or output amplifier stage.

Accordingly, it is an object of the present invention to provide animproved electric signal amplifier embodying electric discharge amplifier devices, and having a power amplifier stage which may selectively be operated as a class A or as a class B amplifier for differing power input or signal levels, whereby the advantages of class A and 1934, Serial No. 738,796

of class B amplification may be taken advantage of in one amplifier.

Two types of class B audio frequency or improved class B amplifiers may be used for a power amplifier stage. In one type, the biased class B amplifier, the grids are biased by potential until the anode current is substantially zero; and involves increasing the normal class A biasing potential to effect the transition, the amplifier circuit being otherwise adapted for class B operation.

In the other type, the zero biased class B amplifier, a plurality of grid electrodes are provided in each tube, and are interconnected with the cathodeto produce a normal no load anode current of desired low or substantially zero value.

In changing to class B operation, the response characteristics of the output stage changes, that is, the amplification factor or gain of the stage is lowered. Therefore, in controlling the signal gain or signal output level of the amplifier, the gain, preferably, is varied to a greater degree at the transition from one class of operation to the other in order to cause the overall gain to vary uniformly throughout the volume control range.

It is, therefore, a further object of the present invention to provide an electric discharge amplifier system having signal volume or gain control means and means conjointly or simultaneously operable therewith for controlling the amplifier 7 operation, whereby the output stage of the system increases in power rating as the signal input level is raised, without 'losingin gain.

It is also a further object of the invention to class B operation in the power handling stage by 7 volume control means in connection with the pre-amplifier or driver stage, and bias control means in connection with'the power amplifier oroutput stage.

It is also a further object of the invention to provide an improved method of and means for tion and continues as a class B amplifier at the higher signal level, the volume control for the amplifier being changed in such a manner that the signal output level or volume of the amplifier is varied substantially uniformly.

A volume control system for audio frequency amplifiers embodying the invention is preferably utilized in connection with automatic volume control systems whereby the audio frequency amplifier is supplied with signals at substantially a constant average amplitude or level.

The invention will be understood from the following description when considered in connection with the accompanying drawing, and its scope will be pointed out in theappended claims.

In the drawing,

Figure 1 is a schematic circuit diagram of an audio frequency amplifier provided with a volume control system embodying the invention,

Fig. 2 is a schematic circuit diagram of a similar amplifier showing a modification of the invention wherein the amplifier is controlled in part by switching means, and

Fig. 3 is also a schematic circuit diagram of a portion of the amplifier of Fig. 2 showing a modification of the switching means.

Referring to Fig. 1, 5 is an output or second stage audio frequency power amplifier comprising a pair of electric discharge amplifier tubes 6, arranged in balanced or push-push relation to each other for class B operation. The amplifier stage is provided with suitable low impedance in put and output transformers and 8 respectively for class B operation, and the output transformer 8 is connected with a. suitable load such as a loud speaker 9.

The output amplifier stage disclosed is of the biased class B type, and biasing potentials are supplied thereto through a cathode return lead indicated at It) and a bias supply lead indicated at ll.

Signals to be amplified are supplied to the output stage from any suitable controllable source such as a pre-amplifier stage including an electric discharge amplifier device 12. The latter device is connected with the input transformer I, and, in turn, receives signals, preferably at substantially constant volume or average amplitude, from signal input terminals I3, through a suitable input transformer Id.

The signal volume as applied to the output stage is preferably continuously variable substantially uniformly from zero to maximum, and in the present example is provided by varying the coupling between the input transformer l4 and the amplifier tube l2 by a potentiometer resistance device 55 connected in shunt across the secondary l 6 of the transformer, and having a movable tap it connected with the input electrode or grid ill of the tube l2. Gridbiasing potentials for the device l2 are supplied through the potentiometer resistor l5 and a lead H] from a suitable source such as a self bias resistor 29 in the cathode return lead 2|.

Other suitable volume control means may be employed in connection with the amplifier to vary the signal potentials applied to the output stage, and in connection with such volume control means, further means are provided for changing from class A operation at low and medium volume levels to class B operation for higher and maximum volume levels. Since the gain of the output amplifier stage is lower and the power handling capacity is higher, for class B operation, the volume control device is preferably arranged to provide a compensating change in volume or gain in the amplifier. To this end the volume control device may be connected for simultaneous operation with means for varying the bias potential on the power stageto provide an increasing biasing potential as the signal voltage on the output stage or gain of the amplifier is increased, whereby the transition from class A operation is gradual. V i

- A definite transition point may also be provided in the volume control adjustment, below which the power amplifier operates class A and above which it operates class B. The volume control means I? of the present example is arranged for the latter method of control and comprises two sections A and B, separated by a short section 0, which is of relatively higher resistance, providing for rapidly increasing or changing the volume over a relatively short range of movement of the contact I1. -This is for the purpose of compensating for the increased power output with loss of gain in changing from class A operation to class B operation of the output amplifier stage.

The transition may occur at a. definite volume or gain level for the amplifier, providing a rapidly changing volume level over a short range of movement of the control contact I! between a low volume range, A, and a high volume range, B, to compensate for the change in power output, whereby the volume control means provides substantially uniform change in volume throughout.

Accordingly, in the present example, the simultaneous bias control for effecting a change from class A to class B operation is provided by a second potentiometer device 22 comprising sections A1, B1, C1, corresponding respectively to the sections A, B, and C of the device l5 and having a movable contact 23 connected as indicated by the dotted line 24 for uni-control operation with the contact I"! by suitable means such as an operating knob, indicated at 25.

In the example shown, the resistor sections A1 and B1 of the potentiometer device 22 are of relatively low resistance providing for no appreciable change in biasing potential, while the resistor section C1 is of appreciable resistance sufficient to increase the bias for class B operation.

The potentiometer 22 is included as a part of a bleeder or potential divider resistor, the remaining sections of which are indicated at 26 and 27, between a high or positive potential supply terminal 28 and a low potential supply or negative terminal 29. The cathode return lead is connected between the sections 26 and 21 whereby the section 26 provides an initial or class A biasing potential for the stage 5. It will be seen that by this arrangement, additional biasing potential for class B operation is included in the grid to cathode circuit I0ll when the contact 23 is moved to the position shown in the B1 range.

' The resulting increase in the bias over the range to provide class B operation of the power stage causes'the power handling capacity to be increased and the gain of the power stage to be lowered. To compensate for lowering the gain in the power stage, the amplifier gain, such as the gain in the first stage, is rapidly increased over the section C so that the overall variation in volume is substantially uniform, in the transition from the volume control range A to the volume control range B and vice versa. This is desirable in order that the resulting control of volume by the device I5-l1 may be smooth throughout the full range of control. To this end, furthermore, the sections A1 and B1 may be tapered in any well known manner.

' The volume control means, therefore, provides for simultaneously controlling the operating characteristic of the power amplifier or output stage, whereby the latter may operate class A for a normal or lower range of signalinput potentials, and increase in power rating rapidly to class B operation, as the volume or signal input potential is raised above a predetermined point, while maintaining a uniform increase in the overall gain of the amplifier, throughout the volume control range. In the present example, this is accomplished by providing 1mi-control operation of the gain in the first stage amplifier and the class of operation of the second stage or output amplifier, by bias control.

In order that the anode potential may also be changed under certain operating conditions, from class A to class B operation, the anode supply circuit indicated at 3 may be arranged for variable supply through a variable tap 3! on the potential divider resistor section 21, as shown. Anode potential for the first stage amplifier is also provided through an anode supply lead 32 from the same source.

For uni-control operation of the volume or signal level applied to the amplifier and of the bias potential applied to the second stage amplifier, the control devices l5l'| and 2223 preferably have the same range of operation between the same limits, as indicated in the drawing, corresponding to minimum and maximum volume. As the particular uni-control connection to be employed does not concern the invention, further description is believed to be unnecessary. V

The amplifier is preferably supplied with audio frequency signals from a source of constant amplitude such as a receiver 33 having automatic volume control means, whereby the supplyterminals I3 receive audio frequency signals of substantially constant average amplitude.

In connection with a receiver having automatic volume control means, or other apparatus providing signals of substantially constant average amplitude, it will be seen that the transition from class A operation to class B operation may more accurately be provided at a definite point in connection with the volume control device.

In the present example the transition from class A to class B operation is provided over a relatively short range of movement of the volume control device and is accomplished by dual means, each portion of which is a potentiometer device and readily adapted for gang or uni-control by means of a commonoperating knob.

In the present example, the power amplifier stage is adapted for class B operation by change in grid bias. In certain types of class B amplifiers, the grid or grids are operated at zero bias, a sufficient number of grids being provided to reduce the plate current substantially to zero without bias and in the absence of received sig- .nals.

Selective control of a power or output amplifier stage of the zero bias class B type, in conjunction with volume control means is shown in Fig. 2, to which attention is now directed.

Referring to Fig. 2, a pair of electric discharge amplifier devices 34 and 35 are arranged in balanced or push-push relation .to each other as a power amplifier stage having an input transformer 33 and an output transformer 31. The power stage is driven by a suitable first stage or pre-amplifierdevice 38 to which signals are supplied from a phonograph pickup device 39 through a suitable coupling transformer 40 and a shunt variable potentiometer 4|.

-. The phonograph pickup device represents any suitable source of signals of substantially constant average amplitude and the signal potential applied to the amplifier is controlled in the usual manner by the volume control potentiometer 4|.

The input and output transformers 36 and 31 for the power amplifier stage are designed for class B operation, providing relatively low input and output impedances in circuit with the devices 34 and 35. The output transformer 31 is utilized to couple the amplifier to any suitable load device (not shown).

Each of the devices 34 and 35 is provided with a cathode 42, an anode 43, and two grid electrodes Hand 45, which, for class B operation, are con-f nected together as a control grid, while for class A operation the grid adjacent to the anode may be connected thereto, and bias potential is provided for the grid 44. Part of this change of connection is provided by suitable switching means 46-41, and, in the present example, is shown as a two-point switch means having positions A and B, corresponding to the class A and class B operation.

As indicated in the drawing, the switches 46 and 41' are gang connected for simultaneous operation with the potentiometer 4|. This connection is indicated by the dotted line 48 in the drawing, and the changeover of the switch contacts is efiected as the contact 4| is moved from the range A into the range B indicated along the potentiometer device 4 I, the switch contacts being connected corresponding to the range of operation of the potentiometer.

Two additional control means are provided in connection with the power amplifier for changing the grid bias on the power amplifier tubes and the plate potential applied thereto for the two dificrent classes of operation. In the present example, for Zero bias class B operation, the grids 44 and the grids 45 are connected together as one, and, through the input circuit, directly to the cathodes 42 through the contact B of a switch 43, thereby shunting a resistor 58 and a by-pass condenser 51 which are connected in the cathode return circuit. The resistor provides a selfbiasing potential for the amplifiers 34 and 35 when the switch 49 is moved to open the contact B and close the contact A. The amplifier is there fore operable as a self-bias class A amplifier using the grids 44 as control grids, and as a zero bias class B amplifier using both grids 44 and. 45 as a control element or grid.

For class B operation it is desirable to increase the anode or plate potential on the power amplifier stage, and to this end a switch 52 is provided in the anode circuit 53 for connecting the anode circuit directly with a positivesource of potential 54 through the contact B of the switch 52, or indirectly therewith, or to a source of lower po-- tential through a resistance element or potential drop-producing device 55 when the switch 52 is moved to close the contact A, thereby providing anode potential of a proper predetermined value However, a switching arthe switches 49 and '52 are readily adapted for gang or uni-control with the switches 46 and 41. It will be noted that the uni-control connection 48 extends through and in connection with the switches 49 and 52, whereby they are simultaneously operable along with the switches 46 and 41 by means of the common control connection with the potentiometer 4! The volume and bias control of the amplifier may therefore be placed under control of a single operating knob as indicated at 56.

It will also be noted that the positive source of potential 54 is connected through a negative terminal 5! with the cathodes of the devices 38, 34, and 35 through a cathode return lead 58, and that the first stage or pro-amplifier 38 receives anode potential from the source through a head 59 and a potentiometer or voltage divider resistor 60in the usual manner.

The operation of the amplifier shown in Fig. 2 is substantially the same as that of the amplifier shown in Fig. 1 except that the transition from class A operation to class B operation for the higher volume levels is effected through switching means actuated by the volume control device. In the present example, the switches are changed over from the contacts A to the contacts B as the potential or volume control device passes a predetermined point such as a point 5| between two corresponding volume control ranges of operation A and B, thereby providing for class B operation to meet the requirement for higher power output as the volume level applied to the amplifier is increased above a predetermined level at which the class A operation would normally be inadequate to handle the power output without distortion.

The volume control device may be arranged in any other suitable manner, but is preferably provided in connection with a signal circuit preceding the power amplifier stage. In an amplifier, as shown, having a pre-amplifier or first stage, the volume may conveniently and effectively be controlled by a simple potentiometer device in shunt with the signal circuit.

Likewise, the volume control device may be provided with any suitable taper, and with an intermediate section, such as shown and described in connection with the circuit of Fig. l, for attaining a smooth and uniform, change in volume or signal level through the transition point from class A to class B operation.

With reference to the above volume control arrangement and a switching arrangement for simultaneously changing the anode and grid bias circuits of a power amplifier, a modification of the circuit of Fig. 2 is shown in Fig. 3, in which the same reference numerals are used to designate the like parts as in Fig. 2.

Referring to Fig. 3, it will be noted that the volume control device 4! is provided with an intermediate impedance or resistance element 6| between the sections A and B, connected between adjacent points 53 on the winding of the potentiometer. The potentiometer 4! is mechanically connected with the switches 46, 41, 49, and 52, as indicated by the dotted connection 48, the arrangement being such that the switches are moved to change the connections as the contact of the potentiometer 4i moves across the points 63. With this arrangement, the volume is rapidly increased or decreased at the transition point by the resistance of the device 6| to compensate for the change in gain of the output or power amplifier stage when changing from class B to class A operation, and vice versa.

It will also be noted in the present example, that the potential reducing resistor 55 is included in the cathode return lead of the output or power stage and between the self-bias resistor 58 and the ground or negative anode supply lead 58, whereby the potential applied to the power amplifier between anodes and cathodes for class A operation, is reduced, and the self-bias potential is provided by impedance means inserted in the cathode return lead for the amplifier. This has the advantage that the switching connections are operated at low voltage, that is, in the negative supply lead, and are not connected with the high potential or positive anode potential supply lead. The operation of the system is otherwise the same as that of Fig. 2, and further description is believed to be unnecessary. 7

From the foregoing description it will be seen that the amplifier system in accordance with the invention provides the high quality or fidelity of a class A amplifier when operated at low and medium signal levels, and the high power handling ability of a class B amplifier when the signal level is raised above a predetermined value such that the class A amplifier would be overloaded. With this arrangement the reserve power output is available when desired and is automatically brought into operation with increases in the applied input signal level under control of the volume control means.

The control arrangement further provides for a change from class A to class B operation by continuously variable means comprising a volume control device for controlling the signal amplitude or voltage applied to a power handling amplifier stage and a bias control device providing, as the volume is varied, for change or variation in the bias potential applied to the power handling stage, either gradually, or rapidly at a selected transition point corresponding to a permissible signal amplitude above which the amplifier stage is preferably class B for increased power output and below which the amplifier is preferably class A for increased fidelity.

I claim as my invention:

1. The combination with an electric discharge audio frequency amplifier, of means for varying the signal potential .applied to said amplifier, means for changing said amplifier from class A operation to class B operation when the signal level is increased above a predetermined value by said. first named means, and operating means interlocked With said first and second named means.

2. The combination with an audio frequency power amplifier comprising a pair of electric discharge devices arranged in balanced relation to each other, of input and output circuits for said devices adapted for class B operation, means for selectively adjusting said circuits whereby said devices are adapted for class A operation, and volume control means for said amplifier connected with said circuit adjusting means to operate the same simultaneously with adjustment of said volume control means at a predetermined point of adjustment.

3. The combination with an audio frequency power amplifier comprising a pair of electric discharge devices arranged in balanced relation to each other, of input and output circuits for said devices adapted for class B operation, means for selectively adjusting said circuits whereby said devices are adapted for class A operation, means for controlling the signal volume applied to said amplifier, and operating means therefor interlocked with said first and second named means,

4. The combination with an audio frequency amplifier comprising a pair of electric discharge devices arranged in balanced relation to each other and provided With input and output circuits adapted for class B operation, of means for controlling the signal volume applied to said amplifier in two differing volume ranges, means providing an initial biasing potential for said amplifier for class A operation through one volume range, means interlocked for operation with said first named means for increasing the biasing potential on said amplifier for class B operation through another volume range, and means associated with said volume control means for compensating for a change in gain of said amplifier imposed by the change of operation thereof.

5. The combination with an audio frequency amplifier comprising a pair of electric discharge devices arranged in balanced relation to each other and provided with input and. output circuits adapted for class B operation, of means for controlling the signal volume applied to said amplifier in two differing volume ranges, means providing an initial biasing potential for said amplifier for class A operation, means interlocked for operation with said first named means for increasing the biasing potential on said amplifier for Class B operation, means associated with said volume control means for compensating for a change in gain of said amplifier imposed by the change of operation thereof, said first named means including a potentiometer volume control device having a low and a high volume control range of operation and including a volume control change section between said low and high volume control ranges.

6. The combination with an audio frequency power amplifier comprising a pair of electric discharge amplifier devices having balanced input and output circuits adapted for separate class A and class B operation, of means for controlling the signal potential applied to said devices 1 through a predetermined low and a predetermined high volume range of operation, and means interlocked for operation with said first named means for causing the direct current component of the anode current of said devices to assume a normal value for class A operation throughout the low volume range of operation and to be reduced to a normal value for class B operation during the high volume range of operation of said signal potential control means.

'7. In a radio receiving system, the combination of signal volume control means therefor, an electric discharge amplifier stage for amplifying signals controlled by said means and adapted for class B audio frequency operation, and means for converting said amplifier stage to class A operation in response to operation of said volume control means in a predetermined low volume range of operation.

8. An audio frequency electric discharge amplifier comprising a first stage amplifier, continuume range of operation, and operating means connected with said interlocked means.

9. An audio frequency electric discharge amplifier comprising a first stage amplifier, volume control means connected therewith, a second stage amplifier adapted for class B audio frequency operation, means for converting said second stage amplifier to class A operationin response to operation of said volume control means in a predetermined low volume range of operation, and said volume and converting means including a pair of continuously variable potentiometer devices connected for uni-control operation throughout the same range of operation, and each being provided with an intermediate section between its ends for effecting a relatively high degree of change of signal potential on said first amplifier stage and bias: potential change on said second amplifier stage.

10. In an electric discharge class B audio frequency amplifier, means for controlling thesignal potential applied thereto, and means connected with said first named means to be operated thereby for simultaneously decreasing the anode potential and increasing the biasing potential for said amplifierwhen said signal potential is decreased below a predetermined level,

11. In an audio frequency amplifier, a power amplifier stage comprisinga pair of electric discharge amplifier devices arranged in balanced relation to each other between input and output circuits adapted for class B operation, said am.- plifier deviceseach having a cathode, an anode, and at least two grid electrodes therebetween, one of which is a control grid, switching means for connecting one of said grid electrodes in each device selectively to the control grid therein for class B operation, and to the anode for class A operation, and means for simultaneously applying a biasing potential tothe control grid and reducing the anode potential, for class A operation.

12. In an audio frequency amplifier, a power amplifier stage comprising a pair of electric discharge amplifier devices arranged in balanced relation toeach other between input and output circuits adapted for class B operation, said'amplifier devices each having a cathode, an anode, and at least two grid electrodes therebetween, one of which is a control grid, switching means for connecting one of said grid electrodes in each device selectively to the control grid for class B operation, and to the anode for class A operation, means for simultaneously applying a biasing potential to the control grid and reducing the anode potential for class A operation, a driver stage for said amplifier, and means associated therewith for controlling signal potentials applied thereto in a low volume range when the power amplifier stage is connected for class A operation and in a high volume range when the power amplifier stage is connected for class B operation.

13. In an audio frequency amplifier, a power amplifier stage comprising a pair of electric discharge amplifier devices arranged in balanced relation to each other between input and output circuits adapted for class B operation, said amselectively to the control grid for class B operation, and to the anode for class A operation, means for simultaneously applying a biasing potential to the control grid and reducing the anode potential for class A operation, a driver stage for said amplifier, means associated therewith for controlling signal potentials applied thereto in a low volume range when the power amplifier stage is connected for class A operation, and in a high volume range when the power amplifier stage is connected for class B operation, and means providing a control connection between said volume control means and said selective switching means for effecting a change from class A to class B operation between said volume control ranges of operation.

14. In an audio frequency power amplifier, a pair of electric discharge amplifier devices each having a cathode, at least one control grid and an anode, a cathode return lead therefor, a pair of resistors in said lead, means providing a connection for said control grid between said resistors, means including a movable volume control element for controlling signals applied to said amplifier, and means for short-circuiting said resistors in response to movement of said volume control element through a predetermined position intermediate the ends of its range of movement.

15. In a radio receiving system having automatic volume control means, the combination of an audio frequency power amplifier, a pair of electric discharge amplifier devices each having a cathode, at least one control grid and an anode, a cathode return lead therefor, a pair of resistors in said lead, means providing a connection for said control grid between said resistors, means including a movable volume control element for controllingsignals applied to said amplifier, and means for short-circuiting said resistors in response to movement of said volume control element through a predetermined position intermediate the ends of its range of movement.

16. In a radio receiving system having automatic volume control means, the combination of an audio frequency power amplifier, a pair of electric discharge amplifier devices each having a cathode, at least one control grid and an anode, a cathode return lead therefor, a pair of resistors in said lead, means providing a connection for said control grid between said resistors, and means for concurrently short-circuiting said resistors and increasing the signal potential on said amplifier above a predetermined level.

17. The combination with an electric discharge class 3 audio. frequency amplifier, of means for controlling the signal potential applied thereto, means connected with said first named means to be operated thereby for simultaneously decreasing the anode potential and increasing the biasing potential for said amplifier when said signal potential is decreased below a predeterminedv level by said first named means, and means for establishing said signal potential at substantially a constant value on said first named means,

whereby a predetermined relationship is maintained betweenfthe value of said anode and biasing potentials and the increased signal potential applied to said amplifier.

WALTER VAN B. ROBERTS. 

